The present disclosure relates to methods of forming semiconductor devices. As the feature size of a Metal Oxide Semiconductor (MOS) transistor decreases, the length of a gate and the length of a channel that is formed below the gate may become shortened. Accordingly, research has been conducted to increase capacitance between the gate and the channel and to improve operation characteristics of the MOS transistor.
As the thickness of a silicon oxide layer that is mainly used as a gate insulating layer is reduced, the electrical properties thereof may approach a physical limit. Accordingly, for replacement of an existing silicon oxide layer, research for a high-k layer having a high dielectric constant has been conducted. The high-k layer can reduce leakage current between a gate electrode and a channel region while maintaining the thickness of a thin equivalent oxide layer.
Further, polysilicon that is mainly used as a gate material may have a higher resistance than the resistance of most metal materials. Accordingly, a polysilicon gate electrode may be replaced by a metal gate electrode.